Generally, when constructing a tunnel of a large bore, a pilot shaft is first driven by a pilot tunneling machine and then reamed to a desired large bore by a reaming tunneling machine.
One known example of such a reaming tunneling machine is shown in FIG. 8. In this figure, a tunneling machine 101 comprises a main beam 104 that extends through a pilot shaft 102 and through a reamed shaft 103 and a main frame 106 that is supported by the main beam 104 so as to be slidable back and forth and supports a cutter head 105 at its front portion. Disposed in front of the main beam 104 is a nose gripper 107 for gripping the main beam 104 against the inner circumferential surface of the pilot shaft 102. Disposed behind the main beam 104 is a main gripper (rear gripper) 108 for gripping the main beam 104 against the inner circumferential surface of the reamed shaft 103. Gripper shoes 113 (see FIG. 9) for the main gripper 108 are coupled to the main frame 106 by means of a plurality of thrust jacks 109. The main frame 106 includes a holding gripper 110 for supporting the main frame 106 against the inner circumferential surface of the reamed shaft 103, upon and after completion of excavation.
Excavating/advancing operation by use of the tunneling machine 101 of the above-described structure is carried out in the following way.
In preparation for starting excavation, the main gripper 108 is extended under high pressure, the holding gripper 110 is held under reduced pressure, and after the nose gripper 107 is extended under high pressure, the holding gripper 110 is contracted. In this condition, excavation is carried out by rotating the cutter head 105 and extending the thrust jacks 109. After the main frame 106 is moved forward a distance corresponding to the stroke of the thrust jacks 109, the cutting head 105 is moved somewhat backwardly from the cutting edge and the rotation of the cutter head 105 is stopped to finish excavation. For gripper replacement, the holding gripper 110 is extended under high pressure, and after the nose gripper 107 is contracted, the main gripper 108 is contracted. After that, the thrust jacks 109 are contracted for thrust jack replacement and a cycle of excavating/advancing operation is thus completed. The cycle of excavating/advancing operation comprising a series of steps of: preparation for excavation; a start of excavation; completion of excavation; gripper replacement; and thrust jack replacement is repeated to drive a tunnel continuously.
As seen from FIG. 9, the main gripper 108 incorporated in the above-described type tunneling machine comprises a carrier 111 supported by the main beam 104, a gripper jack 112 supported by the carrier 111 and gripper shoes 113 supported on the forward end of the rod of the gripper jack 112 with the help of pins. For retaining the gripper jack 112 and the gripper shoes 113 within their specified pivotal ranges, jack retainers 115 each including a coil spring 114 therein are provided in front of and behind the gripper jack 112 respectively, and shoe retainers 117 each including a coil spring 116 therein are provided for the gripper shoes 13.
The tunneling machine having the above structure, however, reveals the following disadvantages during gripper replacement when it is used in excavation of an inclined shaft.
(1) When the main gripper 108 is contracted after the contraction of the nose gripper 107 for gripper replacement, the loads W of the main beam 104, the carrier 111 and a subsequent carriage coupled thereto are all imposed on the gripper jack 112 through the coil spring 114 of the jack retainer 115 positioned behind the gripper jack 112. For this reason, the coil springs 14 need strong force and should be small enough to be inserted between the carrier 111 and the gripper jack 112, and, in consequence, the carrier 111 which supports the coil springs 114 requires more strength to withstand the strong force of the coil springs 114.
(2) After the re-extension of the gripper jack 112 subsequent to gripper replacement, the loads W imposed on the gripper jack 112 continuously works on the coil spring 114 of the rear jack retainer 115 as a compression force and the coil spring 114 is never fled from the compression force during excavation. Accordingly, the spring force of the coil spring 114 is invariably exerted on the gripper jack 112, resulting in damage to the gripper jack 112.
(3) The loads W cause a moment M which allows the gripper shoes 113 to pivot such that their shoe faces confronting the shaft walls turn backward. Because of this moment M, the gripper shoes 113 are inclined so that the gripper shoes 113 cannot be stably pressed against the wall of the shaft.
The present invention has been made with the purpose of overcoming the foregoing problems and one of the objects of the invention is therefore to provide a tunneling machine that is capable of carrying out stable gripper replacement by restricting the loads exerted on the gripper jack for the main gripper as much as possible during the gripper replacement step in excavation of an inclined shaft.